Cobalt-base cast alloys containing such elements as chromium, manganese, vanadium, titanium, tantalum, niobium, boron, hafnium, tungsten, molybdenum, iron, nickel, copper and carbon have generally heat resistance, wear resistance and corrosion resistance superior to those of high-alloy steels, and are therefore popularly in practical use as materials for structural members serving under serious conditions.
In the above-mentioned conventional cobalt-base cast alloys, however, crystal grains of cobalt forming the base grow into coarse dendrites at the time of casting, solidification and cooling thereof. It is also inevitable at the same time that chromium, tungsten and other constituent elements react with carbon, another constituent element, to form carbides which also grow into coarse grains. In addition, as compared with high-alloy steels, the conventional cobalt-base cast alloys are far inferior in the plastic formability in hot and cold. It is therefore very difficult to refine the above-mentioned base crystal grains and carbide grains thus becoming coarse by forging the alloys.
Because of the aforementioned coarse base crystal grains and carbide grains, in putting the conventional cobalt-base cast alloys to practical use, local stress concentration occurring in said coarsening portions often causes breakout of the cast alloys, and moreover, segregation of the constituent elements causes such problems as the decrease in the corrosion resistance of the cast alloys. It is also very difficult to machine the conventional cobalt-base cast alloys into structural members of desired shape and dimensions at a high accuracy because of the very low machinability of such alloys.
With a view to solving the above-mentioned problems inherent to the conventional cobalt-base cast alloys, there is proposed the manufacture of cobalt-base sintered alloys having substantially the same chemical compositions as the cobalt-base cast alloys by the powder metallurgy process.
The known processes now in industrial use for the manufacture of a material powder to be used in the production of the above-mentioned cobalt-base sintered alloy include the electrolytic process, the atomizing process, the milling process and the reducing process.
From among the aforementioned processes for manufacturing a material powder, the electrolytic process is not suitable for the manufacture of an alloy powder, because it is only applicable for the manufacture of a pure metal powder, whereas the atomizing process, the milling process and the reducing process are all suitable for the production of a metal powder or an alloy powder.
However, the alloy powder serving as the material powder manufactured by the conventional atomizing process has not only a spherical shape but also a relatively large particle size, this leading to a low compression-formability. What is worse, in sintering, particle surfaces of said alloy powder are not mutually diffused even by heating said alloy powder to a temperature near the melting point thereof. It is therefore difficult to impart to thus manufactured sintered alloy required properties, especially a high sintered density. Furthermore, the too tight range of sintering temperatures for said alloy powder makes it difficult to apply an effective control over the sintering temperature and hence to go into mass production of sintered alloy.
In the case of the alloy powder produced by the conventional milling process, it is difficult to control the grain size thereof, and inclusion of impurities is inevitable. Furthermore, since said alloy powder is seriously work-hardened by milling, it is necessary to relieve internal stress by the heat-treatment known as normalization prior to its use as the material powder for a sintered alloy.
Although the conventional reducing process is practicable for the production of a metal powder or an alloy powder from relatively easily reducible metal oxide powders, it is very difficult with the conventional reducing process to produce an alloy powder containing such difficultly reducible metal elements as chromium, manganese, vanadium, titanium, tantalum, niobium, boron and hafnium.
Furthremore, the manufacture by the conventional reducing process of alloy powders containing both difficultly reducible metal elements such as chromium, manganese, vanadium, titanium, tantalum, niobium, boron and hafnium and relatively easily reducible metal elements such as cobalt, tungsten, molybdenum, iron, nickel and copper poses the following problems:
(1) Oxide powders of relatively easily reducible metal elements such as cobalt, tungsten, molybdenum, iron, nickel and copper can be easily reduced in a hydrogen atmosphere by heating to a relatively low temperature, whereas oxide powders of difficultly reducible metal elements such as chromium, manganese, vanadium, titanium, tantalum, niobium, boron and hafnium are hardly reduced under the same conditions. PA1 (2) On the other hand, at high temperatures, at which oxide powders of difficultly reducible metal elements such as chromium, manganese, vanadium, titanium, tantalum, niobium, boron and hafnium can be reduced, oxide powders of relatively easily reducible metal elements such as cobalt, tungsten, molybdenum, iron, nickel and copper are naturally reduced, but also, these powders are mutually diffused and sintered, and as a result, become lumpy, thus making it very difficult to recover in the powdery form. PA1 adding, to a main raw material powder comprising cobalt oxide powder, chromium oxide powder and carbon powder, at least one metal oxide powder selected from the group consisting of manganese oxide powder, vanadium oxide powder, titanium oxide powder, tantalum oxide powder, niobium oxide powder, boron oxide powder, hafnium oxide powder, tungsten oxide powder, molybdenum oxide powder, iron oxide powder, nickel oxide powder and copper oxide powder; PA1 mixing and pulverizing these powders to prepare a mixed powder; PA1 reducing said mixed powder by heating under vacuum or in a reducing atmosphere to obtain a reduced sponge-like mass; and PA1 finely pulverizing said reduced sponge-like mass.
Because of these problems, the conventional reducing process cannot be suitable for the manufacture of an alloy powder containing both relatively easily reducible and difficultly reducible metal constituents.